We have made the observation that peripheral T cells expressing V alpha7 genes are highly underrepresented in C57BL/10 mice. This is surprising considering V alpha7 genes comprise 8%-10% of V alpha germline gene pool. The experiments proposed in this grant application seek to determine the mechanism or process which regulates the number of V alpha7-expressing T cells in the periphery. A set os experiments is designed to determine whether the V alpha7 genes carry a structural defect that prevents their rearrangement or efficient transcription. A second set of experiments is designed to estimate the frequence of V alpha7-expressing cells among thymocytes and peripheral T cells; these experiments focus on the potential role of tolerance and/or thymic selection in determining the number of mature, peripheral V alpha7-expressing T cells.